1. Field of the Invention
The present invention relates to a radiation detector and a method for verifying the accuracy of positioning of a radiation detector.
2. Description of the Related Art
A scanning method is being widely used as an irradiation method for a particle therapy. In the scanning method, a target volume is sectioned into fine regions (spots) which are each irradiated with a beam with a small diameter (of 3 mm to 20 mm). After the spot is irradiated with a prescribed amount of radiation, the irradiation is stopped and the next specified spot is scanned with the beam. In order to change a spot to be irradiated in a direction (hereinafter referred to as a lateral direction) perpendicular to a traveling direction (hereinafter also referred to as a depth direction) of the beam, the irradiation position of the beam is changed by scanning magnets. After all spots that exist in a certain depth region are each irradiated with the prescribed amount of radiation, a spot to be irradiated is changed in the depth direction. In order to change the spot to be irradiated in the depth direction, the energy of the beam is changed by an accelerator or a range shifter. Finally, all spots included in the target volume to be irradiated are each irradiated with the same dose.
A proton irradiation system that uses the scanning method is adjusted so that a beam forms a Bragg curve in water for each of spots. This is due to the fact that a main component of a human body to be irradiated by the proton irradiation system is water. The Bragg curve is a distribution of linear energy transfer (LET) (J/m) in the depth direction. The LET is energy that is transferred to a medium for a time period for which a single beam particle travels a unit distance in the medium in the depth direction.
In order to verify the Bragg curve formed after the adjustment, a water phantom in which a radiation detector is installed is used. The radiation detector can be driven by a motor so that the radiation detector moves in the waver phantom in the depth direction. A physical quantity that can be directly measured by the radiation detector is the amount (C) of ionization that occurs in a sensitive region of the radiation detector due to the beam. An absorbed dose D (J/kg) to water is calculated by multiplying the amount of the ionization by a calibration factor. The calculated dose D is an average value in a water equivalent volume of the sensitive region. The water equivalent volume is expressed by multiplying the area S of the medium in the lateral direction by a water equivalent thickness in the depth direction. The water equivalent thickness is a thickness (of water) that causes equivalent energy loss of radiation when the medium is replaced with water. The dose D is expressed by the following equation: D=LET×Φ/ρ, where Φ is the average density (1/m2) of particles incident on the sensitive region, and ρ is the density (kg/m3) of water. In order to convert a distribution of the dose D into the Bragg curve or a distribution of the LET, it is necessary that the average density Φ in the depth direction be constant.
When the number of particles incident on the sensitive region is n, Φ=n/S. When all parts of a beam, which disperse in the lateral direction, can be captured, the number n is constant and Φ is also constant. The radiation detector that is installed in the water phantom and designed to measure a Bragg curve has a sensitive region with a sufficient large area that can capture all the beam parts dispersing in the lateral direction. The water phantom is irradiated with a beam by an irradiation system to be adjusted, the radiation detector is scanned with the beam, and points of a Bragg curve in water are measured on a point basis. In the measurement of the Bragg curve using the water phantom, it takes an immense amount of time to scan the radiation detector.
Non-Patent Document 1 (C. Brusasca, et al., “A dosimetry system for fast measurement of 3D depth dose profiles in charged-particle tumor therapy with scanning techniques” Nucl. Instr. And. Meth. in Phys. Res. B 168 (2000) 578-592) discloses a multilayer radiation detector that measures a Bragg curve of a heavy ion beam at one time. The multilayer radiation detector has a structure in which many ionization chambers are stacked in parallel inlayers in the depth direction. The ionization chambers form the radiation detector that have a structure in which an ionization layer (whose material is rare gas or air) serves as a sensitive region and is sandwiched between two electrodes. A high voltage is applied to one of the electrodes so that a uniform electric field is formed in the sensitive region and then, the ionization chambers collect ionization charges that are generated in the sensitive region during passage of the beam. When a solid phantom that is inserted between the ionization chambers is removed, a measurement interval of a distribution is equal to a water equivalent thickness of a single ionization layer. When it is necessary to reduce the measurement interval, a range shifter is inserted on an upstream side so that the position of a spot to be measured is changed in the depth direction. The measurement interval varies for each of measurement conditions. For example, the measurement interval needs to be 0.2 mm under the condition of a low-energy beam in some cases. In this case, when a water equivalent thickness of a single ionization chamber is 1.0 mm, the thickness of the range shifter is changed to 0.2 mm, 0.4 mm, 0.6 mm and 0.8 mm so that the measurement needs to be repeated five times.
Non-Patent Document 2 (R. Cirio, et al., “Two-dimensional and quasi three dimensional dosimetry of hadron and photon beams with the Magic Cube and the Pixel Ionization Chamber” Phys. Med. Biol. 49 (2004) 3713-3724) also discloses a multilayer radiation detector that measures a Bragg curve of a heavy ion beam at one time. The radiation detector disclosed in Non-Patent Document 2 has ionization chambers that each include two electrodes. One of the two electrodes is divided into 64 strip-like portions. Ionization charges that are generated in a sensitive region are collected for each of the positions of the strip-like portions and added to a Bragg curve so that a size of the beam in the lateral direction in each of layers is calculated.